Sound reproducing circuit for talking pictures and the like



H. B. YAGER July l0, 1934.

SOUND REPRODUCING CIRCUIT FOR TALKING PICTURES AND THE LIKE Filed Sept. 2l, 1931 2 Sheets-Sheet I I IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 1|,l

i l l l l n I l l l l l l I l l I l L INVENTOR. 4f/wey I fw.

H. B. YAGER July 10,` 1934.

SOUND REPRODUCING CIRCUIT FOR TALKING PICTURES AND THE LIKE Filed Sept. 2l, 1931 2 Sheets-Sheet PE. CELL AMPLIFIER TONE CONTROL AND VOLUME CONTROL INPUT TO MAIN AMPLIFIER IOO INVENTOR Hem-9 B. Yager TIIII Patented July 10, 19344 PATENT oEFICE SOUND REPRODUCING 'CIRCUIT FOR TALK- ING PICTURES AND 'THE LIKE Henry B. Yager, New York, N.y Y., assigner, by mesne assignments, to Freeman H. Owens, New

York, N. Y.

Application September 21, 1931, Serial No. 564,128

19 Claims. (Cl. FIB-100.3)

This invention relates to sound. reproducing circuits for talking pictures and the like, and more particularly to a method and means for reducing hum in photo-electric cell sound reproducing equipment when operated from an alternating current source.

It has long been a problem in the use of alternating current operated amplifiers for talking picture and other uses, to eliminate the cycle `and respective harmonic freqency power components in the direct current supply, as rectified, rendered disagreeably audible through the loud speakers, causing noise hereinafter referred to as hum, such hum being incident to the use of, and apparently inherent in amplifiers having an alternating current source of supply.

Particularly has this problem been a troublesome one in amplifiers used for talking picture apparatus where the sound record is a Vphotographic one and the reproduction includes the use of a supersensitive high impedance photoelectric cell circuit.

Many and various kinds and types of rectiers for alternating current have been devised and Vused for rectifying alternating current, thus permitting the use of amplifiers having an alternating current source of supply and eliminating the use of batteries. Such rectiiers, however, even in their present state of perfection fail to entirely do away with the alternating current hum especially in such sensitive circuits as photo-electric cell circuits.

The object of my invention, generally, is` to provide a means and method in connection with the photo-electric cell circuit which reduces vthe alternating current hum and permits the use of an amplifier which is completely powerized from an alternating current source.

More specifically, one object of the present invention is to shunt or bypass ripple or hum components of the rectified and filtered current before the same are applied to the photo-electric cell or/and the photo-electric cell amplifier, for the energy therefrom is subsequently very greatly ampliiied. I have found that this object may be fulfilled by resort to features such as the use of a very low bypass impedance preferably in the form of a shunt condenser of extremely large magnitude, and the localization of the same at or very near the photo-electric cell and its amplifier, in order to prevent pick-up on the powersupply leads at a point beyond the said shunt condenser.

The invention as so far described tends to the prevention of hum by eliminating its cause, but

audible hum may further be prevented by elimination or dissipation of the same at a point following the photo-electric cell amplifier but prior to amplification in the main or power amplifier. 6W In accordance with a further object and feature of the presentinvention, the amplifier system as a whole is provided with bandpass lter means the operation of which may, if desired, be made adjustable so as to at the same time provide means for tone control, and the said filter or tone control is arranged to greatly attenuate and preferably entirely eliminate very low frequencies such as. the power hum and its very low order harmonics. I find that the combination of the means to shunt out ripple components of the rectiiied energy aheadof the photo-electric cell and amplifier, together with the bandpass filter means for eliminating any residual hum, is eX- ceedingly'effective and makes possible the cornmercial reproduction of sound from film with the desired purity and fidelity even when operating the apparatus wholly from an alternating current source. Y

Still another object of the present invention resides in the adaptation and application of the invention as so far described to a photo-electric cell amplifier employing a screen grid tube. The high amplification obtainable when using such a tube in itself reduces the hum or noise level insofar as the same may be due to hum or ripple energy applied to the anode circuit of the tube, and such a modified circuit is for this reason^ additionally effective in avoiding undesired noise or hum despite alternating current operation.

To the accomplishment of the foregoing and such other objects as may hereinafter appear, my invention resides in the circuit elements and their relation one to the other as hereinafter are more particularly described in the specification and sought to be defined in the claims. The specification is accompanied by drawings in which:

Fig. 1 is a schematic wiring diagram showing a rcomplete alternatingY current operated filmY sound reproducing equipment embodying features of my invention; and

Fig. 2 is explanatory of a modification of the invention utilizing a screen grid tube.

MyY invention has been 'illustrated in connection with the translation and audible reproduction of photographic sound records embodying the use of a photo-electric cell.

The reference character 1 refers to an exciting lamp of any suitable construction, and 2 indicatesgenerally an optical slit unit by means of llO which a very narrow beam of light from the lamp 1 is sharply focused as a slit image upon a sound record carried by the photographic film 3 .suitably moved past the point of translation 4 and in alignment with the photo-electric cell 5 adapted to receive the modulated light rays passing therethrough. The details of this translating apparatus form no part of this invention and require no further description.

A source of alternating current on the order of approximately 110 volts is indicated diagrammatically at 6, a switch 7 being provided in the line between the source and a rectifier unit outlined by the dotted line 8. This rectifier unit may be of any desired or known practical type and may include a transformer 9, the current from the secondary of which is rectified by the tube 10 and the rectified current output of which is passed through choke filters 10al and is suitably controlled as at 11.

v In the illustration shown, an 8 volt rectified current is taken from the rectier 8 by means of the wires 12 to supply the current for the eX- citer lamp 1. This same 8 volt rectified current outlet from the rectifier 8 is also used through the wires 13 for the field current of the horn or loud speaker 14.

The photo-electric cell amplifier circuit is shown within a dotted rectangle 15 and actually is contained within a shielded and preferably grounded enclosure which is symbolized by the dotted rectangle 15. This amplifier, generally speaking, maybe of any desired construction and circuit arrangement, and may have any desired number of stages. However, for convenience and compactness, and to minimize undesired and eX- traneous noise and hum the amplifier is preferably made a one stage amplifier. As here shown it comprises a tube 16 the output from which passes through the transformer 17 and thence through a tone control and volume control apparatus shown within the dotted rectangle 18. From the tone control and volume control apparatus the sound energy is applied to the main or power amplifier which is located in the upper portion of the dotted rectangle 19. The tone control and Volume co-ntrol apparatus may, if desired, be of conventional construction, but the tone control apparatus preferably takes the form and is given operating characteristics in accordance with a more detailed description to be given later.

The current for the filament of the tube 16 in the photo-electric cell amplifier is supplied from the 8 volt rectified current line from the rectifier through the wires 12 by means of the wires 20. Any suitable current control such as the rheostat 21 may be provided in this filament circuit.

Referring now to the main amplifier 19, the same is supplied with current from the 110 volt alternating current source which may be the same source as indicated at 6 and in the line of which is provided a switch 22. The current enters the main amplifier through a transformer 23, one secondary 23d of which supplies current which is rectified by a suitable rectifier indicated generally by the tubes 24, the laments of which are heated from secondary 23a, and the rectified current output of which is filtered in a conventional lter, and applied to any suitable voltage divider circuit or resistance 25, in order to provide outlets for rectified alternating currents of different voltages.

A current at 90 volts is taken from this voltage divider through the wire 26 and led. to the.

photo-electric cell amplifier and impressed upon the anode 27 of the photo-electric cell through the wire 28 and resistance 28E, and upon the primary of the transformer 17 through the wire 29 to supply a space current to tube 16. There is provided in the circuit of this 90 volt current between its source and the photo-electric cell and the transformer, a low impedance bypass condenser 30, of high capacity. It is this condenser 30 which results in the substantial elimination of hum in the photo-electric cell and amplifier circuit.

The condenser 30 is of exceptionally large magnitude in order to provide a shunt path of exceedingly low impedance. An ordinary audio frequency bypass condenser has a magnitude of the order of 2 microfarads. The condenser of my invention, however, has a magnitude which, I find, should not be less than 12 microfarads and preferably is equal to or greater than 18 microfarads, extending up to and including a magnitude of 150 microfarads. While theoretically a condenser of even greater magnitude may be desirable, I find that above a value of 150 microfarads the improvement in hum elimination is so slight as not to merit the cost and space of further increase in size. It should also be noticed that condenser 30 is not connected with or associated directly with the main filter following the alternating current rectifier, but instead is localized at the photo-electric cell amplifier itself. As will readily be understood by those versed in the art, the photo-electric cell pick-up and its associated amplifier are located at the projector, whereas the rectifier and power amplifier may be spaced therefrom. At least two projectors are used in practice, and a photo-electric cell and amplifier are used with each projector, whereas only one main amplifier is necessary. In accordance with my inventionk the condenser 30 is located ahead of the photo-electric cell and amplifier so that ripple or hum components of current coming from the rectifier over lead 26 are shunted or by-passed directly to ground through the eX- ceedingly low impedance path provided by the condenser 30, and this shunting of the undesired hum energy to ground takes place before the same reaches either the photo-electric cell or the amplifier tube 16. While the condenser is necessarily located ahead of the cell and amplifier, it is preferably connected as near thereto as is conveniently possible in order to prevent additional pick-up of hum or other undesired noise energy on the lead or conductor 26 running to the cell and its amplifier. In the specific form here shown, the condenser is located within the shield 15 of the photo-electric cell amplifier and is grounded directly to the said shield which in turn is, of course, kept at ground potential. The operation of condenser 30 seems to be based both upon its energy storage or smoothing ability and upon its shunting or bypassing action.

The main amplifier 19 may be of any desired number of stages of amplification and of any desired circuit arrangement, the one illustrated comprising the two stages indicated by the tubes 31, which are impedance coupled, and the tubes 32, which are arranged in push-pull, the output from which is applied to a transformer 33, the secondary of which may be tapped and connected by the wires 34 to a loud speaker, as shown.

Current is supplied to the filaments of the tubes 31 from the filament secondary 23h of the transformer 23, through the wires 35 and 36. Current for the filaments of the tubes 32 is supizo 'sol

plied from the filament secondary 23e of the power transformer 23, through the wires 37. The plate current for the plate of the first tube 31 is supplied through the wire 38 from a 90 volt tap on the voltage divider 25. Current for the plate of the second tube 31 is supplied through the wire 39 from the 135 volt tap of the voltage divider 25. Plate current for push-pull tubes is supplied through wire 40, connected after the first filter section because, on the one hand, an exceedingly high anode potential is desired, and on the other hand, the tubes 32 being the last stage of amplication and being arranged in push-pull are not sensitive to hum or ripple current. The loud speaker 14 may be of the electro-dynamic type, in which case the magnetic field energy may be supplied to the field coil from the 8 volt rectifier through the conductors 13, as shown.

Attention is now directed to the modification of my invention shown in Fig. 2. Inasmuch as the invention centers about the photo-electric cell and amplifier, as Well as the tone control used in conjunction therewith, only these portions of the diagram are shown, thereby permitting an enlarged scale. It will be understood, however, that the photo-electric cell corresponds to the photo-electric cell 5 in Fig. 1, and that it is similarly preceded by an exciter lamp, an optical slit system, and film moving and guiding means.

The 8 volt filament supply through conductors 52 may be obtained from an 8 volt rectifier like that shown in Fig. 1. The main filter 54 correspcnds to the high potential filter shown in Fig. 1, and may be preceded by a similar arrangement of transformers and rectifier tubes. The output from the tone control and volume control unit 56 is fed through conductors 58 to a power amplifier which also may be like that shown in Fig. 1. It will be appreciated, however, that while the circuit arrangement of Fig. 2 has been shown as applicable directly to the general circuit arrangement shown in Fig. 1, Various changes may, if desired, be made. For example, the cathode heating energy supplied through conductors 52 may be alternating or direct, and in either case may be obtained from a main power supply transformer which is combined with that used for the main filter 54 instead of being separated as shown in Fig. 1. The audio frequency energy through conductors 58 may be fed to any desired type of power amplifier, from which the amplified energy may be led to a loud speaker of desired type other than the electro-dynamic type shown in Fig. 1, and if' an electro-dynamic speaker is used, the field may be excited at high potential instead of at low potential as shown at Fig. 1. Such changes are but typical of numerous others which may be made without departing from the essential features of the present invention, next described.

n the present form of the invention the single vacuum tube 60 of the photo-electric cell amplifier is of the screen grid type having a cathode 62, a heater 64 therefor, a control electrode 66, an anode 68, and a screen grid 70. Such a tube is characterized by high gain, and therefore reduces that component of hum caused by the energy supplied to the tube anodefor the desired speech energy is amplified to` agreater relative value than is the case with an ordinary tube. The screen grid tube 60, however, requires a high anode potential of the order of, say, 200 volts, and it is therefore desirable to separate the polarizing energy for the tube anode from the polarizing energy for the photo-electric cell 50 which only requires a potential of the order of volts.

' Accordingly, a lead '72 connected to a 90 volt tap on the voltage divider 74 is led directly to the photo-electric cell 50 and is connected thereto through a resistance 76. The projecting portion 78 of the lead to the cell 50 is preferably shielded, as by the shield 80. The photo-electric cell amplifier is contained within a grounded metallic shield 82, and the lead 72 for the cell 50 is shunted to grounded shield 82 through condenser 86 at point 84. This condenser is made extremely large, as was explained in connection with Fig. l, and is preferably of the order of 50 microfarads. It is preferably localized at the photo-electric cell and amplifier, and if convenient, is contained within the shield 82. If, because of its bulk, as in the case of electrolytic condensers, it is inconvenient so to do, the condenser may be located outside of the shield 82 but as near thereto as is conveniently possible in order to minimize pickup of undesired energy on lead 'I2 between the bypass condenser 86 and the photo-electric cell.

Potential fluctuations corresponding to the light fluctuations impressed on cell 50 are applied to control electrode 66 of vacuum tube 60 through a coupling condenser 88. Coupling resistance 90, current limiting resistance 92, and biasing resistance 94,-audio frequency bypass condensers 96 and 98, and filament rheostat 100 all correspond to the similar circuit elements shown in Fig. 1, and fulfill similar and known functions.

The direct anode potential for tube 60 is tapped from voltage divider 74 by means of a supply lead 102 which is connected to the primary 104 of output transformer 106. Audio frequency bypass condenser V98 completes the output circuit of the tube from the transformer 106 to cathode 62. Screen grid 70 is polarized at a suitable potential, say 75 volts, by a lead '71 connecting the screen grid to the voltage divider 74.

In order to minimize or eliminate ripple or hum components from the potential applied to the tube anode, a choke 108 is connected in series in supply lead 102. This choke is preferably of large, magnitude, say 200 to 300 henrys, and is preferably localized at the photo-electric cell amplifier. In preferred form, the choke 108 is located and mounted within the shield 82, as is shown in the drawings.

The core of choke 108 and the core of transformer 106 may be grounded by connecting the same to shield 82.

It should be understood that, if desired, a bypass condenser may be employed in association of the anode lead for the vacuum tube. It may further be remarked that the use of a condenser and choke, as explained in connection with Fig. 1, may be applied when using an ordinary vacuum tube in contradistinction to a screen grid tube. Furthermore, a choke and a condenser may, if desired, be used on a single supply lead for both the cell and tube in an arrangement such as that shown in Fig. 1. In each case, however, the condenser or/ and choke should preferably be largeV in magnitude and should preferably be localized at the cell and cell amplifier.

The output from transformer 106 is connected `through leads 110 to the tone control and volume control unit 56. The tone control is a form of bandpass filter and is adjusted to pass frequencies in a desired range, but to attenuate frequencies outside of that range. The unit here shown comprises a resistance 112 connected in series with a parallel circuit 114 having an inductance 116 in one branch and a condenser 118 in its other branch. Theentire series combination of microfarads may instead be employed.

the resistance 112 and parallel circuit 114 is connected in shunt across leads 110 by connections indicated at 120 and 122. Resistance 112 is variable, and this permits adjustment of the cutoff frequency of the lter. The unit is preferably designed to freely pass frequencies between, say, 500 and 5000, but to attenuate frequencies below 500, and to completely eliminate frequencies below, say, 220, thereby removing from the audio frequency output the power supply hum frequency and its low order harmonics.

It is important to note that this filter and the condenser or/and choke means for eliminating ripple from the anode supply to the cell or/and cell amplifier tube are best used in combination, for one seems to compensate the deficiencies of the other, and they two in combination result in excellent noiseless reproduction.

The volume control comprises the three resistances 124, 125, and 128 appropriately interconnected and simultaneously varied by adjustment arms indicated schematically at 130, the resistances forming a T network. The output from the volume control network is taken through conductors 58 to a power amplifier, as was previously explained.

It may be well at this point to explain that the diagrams shown in Figs. l and 2 differ from the usual commercial set-up in that in practice two projectors are employed so that the motion picture reels may be presented in continuous sequence. Each projector is provided with sound pick-up means corresponding to the exciter lamp, photo-electric cell and cell amplifier equipment described in connection with Figs. 1 and 2. However, only a single power amplifier, loud speaker and rectier and filter installation is needed, for the photo-electric cell and amplifier units are used only in alternation. Likewise only a single tone control and volume control unit is needed for the two projectors. It will be appreciated, therefore, that two photo-electric cell amplifiers, such as are shown in Fig. 2, may be arranged in parallel and connected in parallel to the conductors 110 and thence into the tone control and volume control. The output from leads 58 goes to a single power amplifier. The rectifier and main filter 54 is a single unit connected in parallel to the two photo-electric cell amplifiers. No special switching mechanism is needed with this arrangement because the sound pick-up at one or the other of the projectors is automatically cut off by a shutter. If a switch system is desired in addition to this, it is merely necessary to cut off the energization of one of exciter lamps.

When two photo-electric cell amplifiers are connected in parallel, as just described, each may be tted with a large bypass condenser 86 and a large choke 108, as shown in Fig. 2. However, it should be remembered that the two projectors are located quite closely together, whereas the power amplifier, rectifier, and loud speaker equipment may be located at considerable distance from the projectors. It is therefore possible to use a single bypass condenser 86 for the 'two projectors, and a single choke 108 for the two projectors. In such case the condenser and choke.A may be located outside of the shield 82, or may be located inside of one of the shields, it being understood that the wiring between the two projectors should be as short and direct as possible, and preferably shielded. Iftwo bypass condensers are used, they may have a value of 25 microfarads each, or a single condenser of 50 If an electrolytic condenser is used, it is sufficiently bulky to make it desirable to locate the same outside of the amplifier shield, and in such case a single 50 microfarad unit may be used for both amplifiers. The chokes 108, however, may be conveniently located within the amplifier shield, and accordingly two such chokes are preferably used, each of the order of 200 or 300 henrys.

It is important to notice that the tone control and volume control unit is a self-contained unit to which only two pairs of leads are connected. one for the input and the other for the output. It is therefore readily possible to locate this unit remotely from the projection room, and, if desired, the tone control and volume control unit may be located in a special box near the audience so that the tone and volume may be accurately gauged in accordance with the value most agreeable to the audience.

A typical set of values for the circuit elements set forth in Fig. 2 are next given by way of exempliiication. The resistance 76 has a Value of 2 megohms; the condenser 88 is 0.006 microfarads; the resistance 90 is 10 megohms; the resistances 92 and 94 have values respectively of 0.05 and 400 ohms; bypass condensers 96 and 98 are 2 microfarads each in capacitance; condenser 86 has a capacitance of 50 microfarads; while choke 108 has an inductance of from 200 to 300 henrys.

In the tone control unit the resistance 112 has a value of 500 ohms; the inductance 116 is 10 millihenrys; while the condenser 118 has a capacitance of 1 microfarad. 1n the volume control unit the resistance 124 has a value of about 2130 ohms, while the resistances 126 and 128 each have a value of about 409.1 ohms.

The mode of constructing and using, and the many advantages of the circuit arrangement of my invention will, it is thought, be apparent from the foregoing detailed description thereof. The invention makes it possible to operate a sound picture system wholly from an alternating current source without the use and inconvenience of' storage batteries or the like. The necessary direct current supply is obtained from rectifier and filter systems of conventional and commercially available type, but any tendency to disagreeable amplincation of residual hum is overcome by the relatively simple expedient of bypassing or shunting ripple components of the rectified and filtered current directly to ground through a very low impedance path or large bypass condenser at a point preceding but relatively closely adjacent to the photo-electric cell and cell amplifier. Any residual hum is attenuated and eliminated by the use in combination with the foregoing apparatus of a tone control filter which serves not only for tone control, but for the desired elimination of residual hum. The tone control unit may be located remotely from the projectors and from the power amplifier and loud speaker equipment. Only a single tone control and volume control unit is needed for the two projectors commonly employed, and nevertheless the desired control of tone and volume is obtained while the audio frequency energy is of low power and prior to power amplificatiom. so that relatively compact and economical units may be used for the tone control and volume control. The photo-electric cell amplifier may consist of a single screen grid tube, in which case the hum introduced at the anode of the tube is minimized by reason of the high gain or amplification factor of the tube. This hum may further be reduced by the use of a series choke of large magnitude also preferably localized at the photo-electric cell amplifier and preferably within the shield surroundingthe same.

It will be apparent that while I have shown and described my invention in preferred forms, many changes and modifications may be made in the structures disclosed without `departing from the spirit of the invention, defined in the following claims.

I claim:

1. In the operation of sound reproducing equipment including a photo-electric cell, means for reproducing sound controlled by said cell, a source of alternating voltage, and a rectier and grounded lter operated from said source of alternating voltage to supply a polarizing potential to said cell, said iilter including chokes and condensers, the method of very greatly reducing variations in said potential inforder to eliminateA audible hum, which includes additionally shunting ripple components of the rectified and iiltered current directly to ground through a very low impedance path at a point preceding but closely adjacent the photo-electric cell.

2. In the operation of sound reproducing equipment including a photo-electric cell, means for reproducing sound controlled by said cell, a source of alternating voltage, and a rectier and grounded lter operated from said source of alternating Voltage to supply a polarizing potential to said cell, said filter including chokes and condensers, the method of very greatly reducing Variations in said potential in order to eliminate audible hum, which includes additionally shunting ripple components of the rectified and filtered current directly to ground through a very low impedance path at a point preceding butY closely adjacent the photo-electric cell, and thereafter ltering the audio frequency output to attenuate and eliminate residual undesired hum frequencies.

3. Sound reproducing equipment comprising a photo-electric cell, a source of alternating voltage, a rectifier and iilter operating from said source of alternating voltage tosupply a polarizing potential to said cell, said filter including chokes and condensers, and an additional bypass condenser many times the order oi' magnitude of conventional audio frequency bypass condensers arranged to shunt ripple components from the polarizing potential supply lead, said condenser being at least 12 microfarads in capacity.

4. Sound reproducing equipment comprising a photo-electric cell, a Ysource of alternating Voltage, a rectifier andfilter operating from said source of alternating voltage to supply a polarizing potential to said cell, said lter including chokes and condensers,A and a bypass condenser in addition to the lter condensers and arranged to shunt ripple components from the polarizing potential supply lead'at a point preceding but closely adjacent the photo-electric cell.

`5. Sound reproducing equipment comprising a photo-electric cell, a source of alternating Voltage, a rectifier and grounded filter operating from said source of alternating voltage to supply a polarizing potential to said cell, said iilter including chokes and condensers, and an additional bypass condenser many times the order of magnitude of conventional audio frequency L bypass condensers connected between the polarizing potential supply lead and ground at a point preceding but closely adjacent the photoelectric cell, said condenser being at least 12 microfarads in capacity.

6. Sound reproducing equipment comprising a source of illumination, aprieto-electric cell, a photographic record of sound, means to cause said .record to travel between said source and said cell, a source of alternating voltage, a rectier and grounded filter operating from said source of-alternating voltage to supply a polarizing potential to said cell, said lter including chokes and condensers, and a bypass condenser of the order of microarads capacitance connected between the polarizing potential supply lead and ground at a point preceding but closely adjacent the photo-electric cell. Y

'7. Sound reproducing equipment comprising a photo-electric cell and photo-electric cell amplier, a source of alternatingcurrent, rectier and filter means for obtaining anode current for the cell and amplier, said lter including chokes and condensers, and an additional large bypass condenser many times .the order of magnitude of conventional audio frequency bypass condensers arranged to shunt ripple components from the anode supply lead, said condenser being at least 12 microfarads in capacity.

8. Sound reproducing equipment comprising a photo-electric cell and photo-electric cell ampliiier, a source of alternating current, rectiiier and iilter means for obtaining anode current for the cell and ampliiier, said filter includingchokes and condensers, and a bypass condenser in addition tothe lter condensers and arranged to shunt ripple components from the anode supply lead at a point preceding but closely adjacent the photo-electric cell ampliiier.

9. Sound reproducing equipment comprising a photo-electric cell and photo-electric cell amplifier, a source of alternating current, rectifier and grounded i'llter means for obtaining anode current for the cell and amplier, said filter including chokes and condensers, and an additional large bypass condenser many times the order of magnitude of conventional audio frequency bypass condensers connected between the anode supply lead andA ground at a point preceding but closely adjacent the photo-electric cell amplifier in order to shunt ripple components of the rectified and ltered current directly to ground, said condenser being at least 12 microfarads in capacity. x

10. Sound reproducing equipment comprising a source of illumination, a photo-electric cell and photo-electric cell amplier, a photographic record of sound, means to cause said record to travel between said source and said cell, a source of alternating current, rectiiier and grounded lter means for obtaining anode current for the cell and amplifier, said lter including chokes and condensers, and a large bypass condenser of the orderof 150 microfarads in capacitance connected between the anode supply lead and ground at a point preceding but; closely adjacent the photo-electric cell amplii'er in order to. shunt ripple components of the rectiiied and filtered current directly to ground.

11. Sound reproducing equipment comprising a photo-electric cell, a cell amplier tube, a source of alternating current, a rectier and filter providing anode current for the cell and tube, said lter including chokes and condensers, and in addition a condenser and a choke of large magnitude associated with the energy supply leads for the cell and tube anodes, said condenser and choke being localized at the cell amplifier and serving to `minimize audible hum.

12. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier including a vacuum tube, a source of alternating current, a rectifier and lter providing anode current for the cell and tube, said filter including chokes and condensers, and in addition a large condenser many times the order of magnitude of conventional audio frequency bypass condensers arranged to shunt ripple components from the supply lead for the cell anode, said condenser being at least 12 microfarads in capacity, and means associated with the supply lead for the anode of the tube and localized at the cell amplifier for eliminating hum from the energy supply to the tube anode.

13. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier including a screen grid tube, a source of alternating current, a rectier and grounded filter providing anode current for the cell and tube, said lter including chokes and condensers, and in addition a large condenser many times the order of magnitude of conventional audio frequency bypass condensers connected between the supply lead for the cell anode and ground at a point preceding but near the cell, said condenser being at least l2 microfarads in capacity, and means associated with the supply lead for the anode of the tube and localized at the cell amplier for eliminating hum from the energy supply to the tube anode.

14. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier including a Vacuum tube, a source of alternating current, a rectifier and lter providing anode current for the cell and tube, said lter including chokes and condensers, and in addition a large condenser many times the order of magnitude of conventional audio frequency bypass Vcondensers arranged to shunt ripple components from the supply lead for the cell anode, said condenser being at least 12 microfarads in capacity, and a choke i of large inductance connected in series with the supply lead for the anode of the tube and localized at the cell amplifier, said condenser and choke serving to minimize audible hum from the cell and cell amplifier.

15. Sound reproducing equipment comprising a photo-electric cell, a cell amplier including a screen grid tube, a source of alternating current, a rectifier and grounded iilter providing anode current for the cell and tube, said lter including chokes and condensers, and in addition a large condenser many times the order of magnitude of conventional audio frequency bypass condensers connected between the supply lead for the cell anode and ground at a point near the cell,

' said condenser being at least 12 microfarads in capacity and a choke of large inductance connected in series with the supply lead for the anode of the tube and localized at the cell amplifier, said condenser and choke serving to minimize audible hum from the cell and cell amplier.

16. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier including a screen grid tube, a source of alternating current, a rectier and grounded filter providing anode current for the cell and tube, said lter including chokes and condensers, and in addition a condenser of the order of 50 microfarads capacitance connected between the supply lead for the cell anode and ground at a point near the cell, and a choke of the order of 200 to 300 henrys connected in seiies with the supply lead for the anode of the tube and localized at or within the shield of the cell amplifier, said condenser and choke serving to minimize audible hum from the cell and cell amplifier.

17. Sound reproducing equipment comprising a photo-electric cell, a cell amplier tube, a source of alternating current, a rectiiier and filter providing anode current for the cell and tube, said filter including chokes and condensers, means including a condenser and choke of large magnitude associated with the supply lead for the cell and tube anodes and localized at the cell amplier for minimizing audible hum caused by the rectiiier and filter, and a tone control filter circuit following said cell amplifier and arranged to attenuate and eliminate residual undesired hum frequencies, said latter filter including a series combination of a resistance and a parallel branch circuit of inductance and capacitance all connected in shunt across the output from the cell amplifier.

18. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier tube, a source of alternating current, a rectier and grounded filter providing anode current for the cell and tube, said iilter including chokes and condensers, a condenser of large magnitude connected between the supply lead for the cell anode and ground at a point near the cell, a choke of large magnitude connected in series with the supply lead for the anode of the tube and localized at the cell amplier, and a tone control filter circuit following said cell amplifier and arranged to attenuate and eliminate residual undesired hum frequencies.

19. Sound reproducing equipment comprising a photo-electric cell, a cell amplifier including a screen grid tube, a source of alternating current, a rectifier and grounded lter providing anode current for the cell and tube, said lter including chokes and condensers, a condenser of the order of 50 microfarads capacitance connected between the supply lead for the cell anode and ground at a point near the cell, a choke of the order of 200 to 300 henrys connected in series with the supply lead for the anode of the tube and localized at the cell amplier, and a tone control filter circuit following said cell amplier and arranged to at tenuate and eliminate residual undesired hum frequencies.

HENRY B. YAGER. 

